For many years weaving has been the principal method of constructing fabrics for use on paper machines. In the case of papermaker felts, some success has been achieved with needle punched nonwoven felts of the "fillingless" type. See, for example, U.S. Pat. No. 3,392,079, incorporated herein by reference. Such felts are made by winding spaced apart parallel machine direction yarns around tension rollers, covering the yarns with fiber batt, and needling the batt into the yarns from both sides to form a consolidated nonwoven felt.
Under low press loadings such fillingless felts operate satisfactorily; however, under high press loadings fillingless felts are not satisfactory because the machine direction yarns leave undesirable indentations in the paper sheet. Furthermore, such fillingless felts are prone to widening during operation on the paper machine. Thus, such felts are seldom used on highly loaded presses.
Although nonwoven felts are known, it has not been hitherto possible to make paper machine forming fabrics without weaving. The idea of making a satisfactory forming fabric by needling batt fibers into a parallel array of yarns in the same manner as fillingless felts is not practical because such a product would lack sufficient surface uniformity and drainage, would tend to fill up with fines, and would interfere with release of the sheet after formation. Nevertheless, the concept of producing a forming fabric without weaving, has been the subject of continuing interest due to the high cost of manufacturing woven forming fabrics by present methods.
In the case of paper machine forming fabrics, present trends are toward multi-layer woven fabrics having coarse woven substrates made of thick yarns in the wear side of the fabric, with small yarns woven in fine mesh in the sheet forming side of the fabric. The coarse yarns are used to impart wear resistance to the fabric, while the fine mesh surface is used to achieve good sheet formation. It has not been possible to make a woven forming fabric with a smooth sheet forming surface using the same coarse yarns needed to achieve the desired wear properties. The multi-layer fabrics, while providing both smooth surfaces and good wear properties, are difficult and time-consuming to manufacture due to their structural complexity.
Nonwoven fabrics and the manufacture thereof are well known. For example, U.S. Pat. Nos. 4,259,399 and 4,285,748, both of which are incorporated herein by reference, disclose the preparation of nonwoven fabrics. However fabrics prepared therein are not suitable for the applications contemplated here.
With regard to papermakers felts having porous incompressable woven substrates capped with porous fiber layers, known as batt on mesh felts, such felts require sufficient batting to reduce the extremes of pressure that would otherwise occur over fabric knuckles or weave crossovers. This is necessary since otherwise uneven pressure would reduce press dewatering efficiency and might have an adverse effect upon sheet quality.
When prior art woven felts are made with incompressable monofilaments, such base fabrics invariably contain interconnected water flow passageways between yarns. Such passageways allow water to flow forward in the machine direction through the felt base fabric while the felt and paper sheet are subjected to hydraulic and mechanical pressure in the press nip. Such transverse water flow forward in the nip may contribute to increased sheet moisture and reduced pressing efficiency.
Transverse passageways in the woven base fabric also provide channels for the entry of air into the expanding sheet and felt as they leave the press nip. It is suspected that such air entry into the incompressible woven felt substrate may facilitate water transfer from within the sheet-felt interface back into the expanding paper sheet as it emerges from the press nip, thereby reducing press efficiency.
In the manufacture of filtration fabric belts for sludge concentration as well as other purposes, it is often desired to produce a fabric having many small openings, made from yarns that are large enough to provide long service life. Where such yarns are thick monofilaments, they are difficult to weave close enough together to provide the fine mesh openings desired. A compromise is often necessary, the compromise involving a trade-off to smaller yarns so that the desired mesh can be woven.
A nonwoven paper machine dryer fabric comprised of plastic monofilament spirals is also known. Each spiral is joined to its neighbor by means of a pin inserted through the intermeshed loops of the adjoining spiral to form an endless porous dryer fabric belt. As a result of this joining method, such belts are susceptible to failure should any one of the thousands of spiral connecting pins fail during operation on the paper machine.
Porous plastic sheets are known where holes are formed in the plastic sheet during formation, such as by extrusion of two sets of filaments at right angles to each other followed by fusing them together at crossover points. Such sheets are comprised of unoriented polymer material, and if they are produced in the fineness needed for the papermaking application, they would lack sufficient dimensional stability to operate as endless belts on paper machines. This type of material may be subjected to stretching to orient the filaments to achieve increased stability, but at the price of causing the spaces between members to increase beyond the fineness needed for certain applications.
Dimensionally stable plastic sheet material may be rendered porous by means of perforating, drilling, or the like; however, such fabrication methods preclude the use of combinations of materials that are particularly suited for specific tasks. For example, machine direction stretch resistance may best be satisfied with high modulus materials, whereas, cross-machine direction needs may call for materials that are resilient and of lower stiffness.